An automobile body includes various structural members (for example, a rocker (also referred to as a side sill), a cross member, a side member, a center pillar, and the like). For the structural members, press-formed products of steel plates are frequently used. The press-formed products for use in the structural members have channel-shaped or hat-shaped sectional shapes, for example. A press-formed product having a channel-shaped section includes a top plate part and a pair of flange parts connecting to the top plate part. A press-formed product having a hat-shaped section includes a top plate part, a pair of vertical wall parts connecting to the top plate part, and a pair of flange parts connecting to the respective vertical wall parts. In general, mutual edge portions of a plurality of press-formed products are laid on one another, and are joined by resistance spot welding. In this way, a hollow structural member is assembled.
In an automobile structural member as above, reinforcing members are joined by resistance spot welding to important parts such as those where the formed products are joined to each other, and those where high loads are applied. For the reinforcing members, press-formed products of steel plates are frequently used. In the case of a rocker, for example, reinforcing members that partition an inside of the rocker are disposed at one spot or a plurality of spots in a lengthwise direction of the rocker. This is for ensuring three-point bending crush performance of the rocker with respect to the load of a lateral collision. The reinforcing member is referred to as a bulkhead or a stiffener.
FIG. 1 is a perspective view showing an example of a rocker including a conventional bulkhead inside. FIG. 1 shows a state in which a bulkhead 110 inside is seen through. In the following explanation, the case where the structural member is a rocker 1 is taken as an example.
As shown in FIG. 1, the rocker 1 is a long hollow cylindrical body. A sectional shape of the rocker 1 illustrated in FIG. 1 is substantially rectangular. The bulkhead 110 includes a top plate part 111 and flange parts 114. An outline shape of the top plate part 111 substantially corresponds to an inner sectional shape of the rocker 1. The flange parts 114 are formed at respective four sides 112 of the top plate part 111. The bulkhead 110 is disposed in a predetermined position in a lengthwise direction in the inside of the rocker 1. The respective flange parts 114 of the bulkhead 110 are overlaid on inner surfaces of the rocker 1, and are joined to the rocker 1 by resistance spot welding or the like. From the viewpoint of reducing the weight of an automobile body, press-formed products formed from a high-tensile strength steel plate are used for the rocker 1 and the bulkhead 110. Tensile strength of the high-tensile strength steel plate in this case is usually set at 440 MPa or more, and is sometimes set to be more than 440 MPa in accordance with required performance or the like.
Conventionally, the flange parts 114 have been formed by individually folding respective edge parts of the four sides 112 of the top plate part 111. Hereinafter, the individual flange parts 114 which extend from the respective sides 112 of the top plate part 111 will also be referred to as “side flange parts”. In this case, the flange parts 114 are separated to be discontinuous at corner parts 113 that connect the adjacent sides 112 of the top plate part 111. In some cases, cutouts 117 are provided at both side ends of each of the side flange parts 114, as shown in FIG. 1.
Speaking from a performance aspect of the bulkhead 110 (the reinforcing member) in the rocker 1 (the automobile structural member), it is more desirable that the flange parts are configured as follows, than the flange parts 114 are discontinued in the corner parts 113 of the top plate part 111 as in the conventional bulkhead 110 shown in FIG. 1, from an idealistic viewpoint. The flange parts also extend from the respective corner parts 113 of the top plate part 111. Hereinafter, the individual flange parts which extend from the respective corner parts 113 of the top plate part 111 will also be referred to as “corner flange parts”. The adjacent side flange parts are connected via the corner flange part. In this case, the flange parts continue from the certain side 112 of the top plate part 111 throughout the corner parts 113 and the adjacent sides 112. Hereinafter, the flange part that continues without being separated in the corner part 113 of the top plate part 111 will also be referred to a “continuous flange part”.
However, it is very difficult to obtain the product including the continuous flange part by press forming. It has been, of course, conventionally possible to obtain a product such as a lid of a can by press-forming a soft plate material of soft steel, aluminum or the like. However, it is difficult to produce a product including the continuous flange part by especially press-forming a high-tensile strength steel plate, in order to obtain a reinforcing member in the automobile structural member described above. This is for the following reason.
When producing the product including a continuous flange part from a steel plate by press forming, forming of the corner flange part is shrink-flanging. Consequently, wrinkles easily occur in the corner flange parts. The wrinkles occur more remarkably as strength of the steel plate is higher.
When wrinkles occur in the corner flange part, the following problem arises. Referring to the example shown in FIG. 1, when the bulkhead 110 is disposed inside the rocker 1 and mutual overlapping portions are joined by resistance spot welding, gaps due to wrinkles are generated in the overlapping portions. Thereby, a trouble such as poor welding easily occurs. Further, position of the bulkhead 110 in the rocker 1 easily becomes unstable. Accordingly, if wrinkles occur in the corner flange parts of the bulkhead 110, the bulkhead 110 cannot be used as an actual product.
Prior arts that suppress occurrence of wrinkles in the regions to be shrink-flanged at the time of press-forming steel plates are as follows.
For example, Japanese Patent No. 2554768 (Patent Literature 1) and Japanese Patent Application Publication No. 07-112219 (Patent Literature 2) each disclose a technique of forming a roof panel having an opening for a sunroof. Each of the techniques of Patent Literatures 1 and 2 indicates that in order to absorb an excess line length of the region to be shrink-flanged at a time of forming, an excess thickness part is given in advance. Japanese Patent No. 2560416 (Patent Literature 3) discloses a technique of square cylinder draw forming. In the technique of Patent Literature 3, a specific shape is given to a region to be shrink-flanged. Japanese Patent Application Publication No. 04-118118 (Patent Literature 4) discloses a technique of press forming using a cam structure. It is indicated that in the technique of Patent Literature 4, forming is performed while presser bar pressure is given to a region to be shrink-flanged.